Field of the Invention
The present invention relates to an image display apparatus which uses fluorescent light emitted from an excited phosphor, and in particular, to the one suitable for an image projection apparatus (liquid crystal projector or the like) which illuminates an image display element such as a liquid crystal panel provided on an illuminated surface, and projects light emitted from the image display element onto a projected surface such as a screen.
Description of the Related Art
In recent years, various projectors are proposed which have each such a structure as to expand a light flux which has been modulated so as to correspond to image information, by using an image display element such as a liquid crystal display element, and project the expanded light flux onto a screen or the like, through a projection optical system (projection lens system).
As for a light source device (light-emitting device) which is used for a projector that uses a liquid crystal display element, conventionally, a device has been used which takes in a light flux that is emitted from a mercury lamp or the like in all directions, through a paraboloid reflector. Not only such a device, but also a light source (hereinafter referred to as solid light source) is starting to be used which uses a semiconductor such as a light-emitting diode (LED) and a laser, and emits a short-wavelength light (excitation light).
In addition, there is also a method of irradiating a phosphor with light emitted from the solid light source, and utilizes light (fluorescent light) of which the wavelength has been converted. Furthermore, because the intensity of light emitted from the solid light source increases due to the increase of light output, and the phosphor generates heat, a method of reducing the heat generation by rotating a disc-like wheel (hereinafter referred to as phosphor wheel) is known onto which the phosphor has been applied. Here, light conversion efficiency varies due to the unevenness of the thickness of the phosphor that has been applied onto the phosphor wheel, and/or the nonuniformity of the irradiated positions, which originates in nonuniform rotation, and fluctuates in a rotation cycle. However, the light conversion efficiency is averaged if the rotation frequency is sufficiently increased, and accordingly when the rotation frequency exceeds 60 Hz, for instance, the fluctuation is not almost perceived.
However, when the irradiation intensity of the solid light source is changed with time, a beat frequency component is generated which has a lower frequency than the rotation frequency, by the synthesis of the rotation cycle of the phosphor wheel and the cycle of the intensity change of the solid light source. This beat frequency component occasionally degrades a quality of a displayed image as a flicker which degrades a display quality, though depending on the intensity.
United States Patent Application publication No. 2013/0229631 discloses a technique of reducing the occurrence of the flicker by matching the flicker frequency of the solid light source with the rotation frequency of the phosphor wheel or limiting the flicker frequency to the multiples of the rotation frequency, in order that the beat frequency component becomes not less than 20 Hz or does not occur.
Incidentally, Japanese Patent Application Laid-Open No. 2012-155059 discloses a pseudo-impulse driving of which the displayed image is made to resemble an impulse type display, by performing intermittent lighting for interrupting lighting, only in a particular period in one frame period, as black insertion for enhancing an image display performance.